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Abstract:

A MTJ for a spintronic device is disclosed and includes a thin seed layer
that enhances perpendicular magnetic anisotropy (PMA) in an overlying
laminated layer with a (Co/X)n or (CoX)n composition where n is
from 2 to 30, X is one of V, Rh, Ir, Os, Ru, Au, Cr, Mo, Cu, Ti, Re, Mg,
or Si, and CoX is a disordered alloy. A CoFeB layer may be formed between
the laminated layer and a tunnel barrier layer to serve as a transitional
layer between a (111) laminate and (100) MgO tunnel barrier. The
laminated layer may be used as a reference layer, dipole layer, or free
layer in a MTJ. Annealing between 300° C. and 400° C. may
be used to further enhance PMA in the laminated layer.

Claims:

1. A magnetic element, comprising: (a) a seed layer comprising one or
more of Hf, NiCr, and NiFeCr that enhances perpendicular magnetic
anisotropy (PMA) in an overlying laminated layer; and (b) the laminated
layer having intrinsic PMA and comprising a multilayer stack of two
metals, a metal and alloy, or two alloys represented by (A1/A2)n
where A1 is a first metal or alloy, A2 is a second metal or alloy
selected from X, CoX, and FeX where X is one of V, Rh, Ir, Os, Ru, Au,
Cr, Mo, Cu, Ti, Re, Mg, and Si, CoX is "a disordered alloy, and n is the
number of laminates in the stack, the laminated layer contacts a top
surface of the seed layer, or the laminated layer has an (A1/C/A2)
configuration where C is a non-magnetic spacer.

2. The magnetic element of claim 1 wherein the seed layer and laminated
layer are formed in a magnetic tunnel junction (MTJ) having a seed
layer/reference layer/tunnel barrier/free layer configuration, the
laminated layer is part of the reference layer.

3. The magnetic element of claim 2 wherein the reference layer further
comprises a magnetic layer formed between the laminated layer and the
tunnel barrier to increase a magnetoresistive (MR) ratio in the MTJ, the
magnetic layer interfaces with the tunnel barrier and has PMA with a
magnetic moment in a same direction as the PMA in the laminated layer.

4. The magnetic element of claim 1 wherein the seed layer and laminated
layer are formed in a MTJ having a reference layer/tunnel barrier/free
layer/spacer/seed layer/ dipole layer configuration and the laminated
layer is the dipole layer.

5. The magnetic element of claim 1 wherein the seed layer and laminated
layer are formed in a MTJ having a seed layer/composite free layer/tunnel
barrier/reference layer configuration and the laminated layer is part of
the composite free layer.

6. The magnetic element of claim 5 wherein the composite free layer
further comprises a magnetic layer formed between the laminated layer and
the tunnel barrier to increase the magnetoresistive (MR) ratio in the
MTJ, the magnetic layer interfaces with the tunnel barrier and has PMA
with a magnetic moment in the same direction as the PMA in the laminated
layer.

7. A magnetic tunnel junction (MTJ), comprising: (a) a seed layer formed
on a substrate and comprising one or more of Hf, NiFeCr, and NiCr that
enhances perpendicular magnetic anisotropy (PMA) in an overlying
laminated layer; (b) a reference layer having a laminated structure with
a plurality of "n" layers that are represented by (Co/X)n or
(CoX)n where X is one of V, Rh, Ir, Os, Ru, Au, Cr, Mo, Cu, Ti, Re,
Mg, and Si, CoX is a disordered alloy, and the reference layer contacts a
top surface of the seed layer and has intrinsic PMA.

8. The MTJ of claim 7 wherein the seed layer has a thickness between
about 10 and 300 Angstroms.

9. The MTJ of claim 7 further comprised of a magnetic layer made of
CoFeB, CoFe, or combinations thereof that contacts a top surface of the
laminated structure.

10. The MTJ of claim 9 wherein the magnetic layer consists of CoFeB and
the MTJ is further comprised of a Ta layer about 0.5 to 3 Angstroms thick
that is formed between the top surface of the laminated stack and the
magnetic layer.

11. The MTJ of claim 7 wherein n is from 2 to 30.

12. A magnetic tunnel junction (MTJ), comprising: (a) a seed layer formed
on a substrate; (b) a free layer comprising a laminated stack that
contacts a top surface of the seed layer, the laminated layer has
intrinsic perpendicular magnetic anisotropy (PMA) and comprises a
(Co/X)n, or (CoX)n structure wherein X is one of V, Rh, Ir, Os,
Ru, Au, Cr, Mo, Cu, Ti, Re, Mg, and Si, CoX is a disordered alloy, and n
is the number of laminates in the stack; (c) a tunnel barrier layer
formed on the free layer; (d) a reference layer formed on the tunnel
barrier layer; and (e) and a capping layer formed on the reference layer.

13. The MTJ of claim 12 wherein the seed layer is Hf, NiFeCr, or NiCr, or
a composite with a Hf/NiCr, Hf/NiFeCr, NiFeCr/Hf, or NiCr/Hf
configuration and having a thickness between about 5 and 100 Angstroms.

14. The MTJ of claim 12 further comprised of a magnetic layer made of
CoFeB, CoFe, or combinations thereof formed between the laminated stack
and the tunnel barrier layer.

15. The MTJ of claim 12 wherein n is from 2 to 30.

16. A magnetic tunnel junction (MTJ), comprising: (a) a reference layer
formed on a substrate; (b) a tunnel barrier layer contacting a top
surface of the reference layer; (c) a free layer formed on the tunnel
barrier layer; (d) a first non-magnetic spacer contacting a top surface
of the free layer; and (e) a dipole layer contacting a top surface of the
non-magnetic spacer, the dipole layer has a lower underlayer comprising
one or more of Hf, NiFeCr, and NiCr, and an upper laminated layer that
has intrinsic perpendicular magnetic anisotropy (PMA) and has a
composition represented by (Co/X)n or (CoX)n structure wherein
X is one of V, Rh, Ir, Os, Ru, Au, Cr, Mo, Cu, Ti, Re, Mg, and Si, CoX is
a disordered alloy, and n is the number of laminates in the stack.

17. The MTJ of claim 16 wherein the underlayer is Hf, NiFeCr, NiCr, or a
composite with a Hf/NiCr, Hf/NiFeCr, NiFeCr/Hf, or NiCr/Hf configuration
and having a thickness between about 5 and 100 Angstroms.

18. The MTJ of claim 16 wherein the first non-magnetic spacer is Ta with
a thickness from about 0.5 to 2 Angstroms.

19. The MTJ of claim 16 wherein n is from 2 to 30.

20. A domain wall motion device, comprising: (a) a first stack comprising
a lower seed layer and a laminated layer formed thereon wherein the first
stack has a first width and wherein the seed layer is one or more of Hf,
NiCr, and NiFeCr, and the laminated layer has intrinsic PMA and a
composition represented by (Co/X)n or (CoX)n structure wherein
X is one of V, Rh, Ir, Os, Ru, Au, Cr, Mo, Cu, Ti, Re, Mg, and Si, CoX is
a disordered alloy, and n is the number of laminates in the stack; and
(b) a second stack with a tunnel barrier/free layer/capping layer
configuration and having a second width substantially greater than the
first width, the tunnel barrier contacts a top surface of the first
stack.

22. The domain wall motion device of claim 20 further comprised of a
magnetic layer made of CoFeB, CoFe, or combinations thereof that is
formed between the laminated layer and the tunnel barrier layer, the
magnetic layer has PMA with a magnetization in the same direction as the
PMA in the laminated layer.

23. The domain wall motion device of claim 20 wherein n is from 2 to 30.

24. The domain wall motion device of claim 20 wherein the free layer is
part of a wire in an array of wires that is used to store digital
information.

25. A magnetic element, comprising: (a) a seed layer comprising one or
more of Hf, NiCr, and NiFeCr that enhances perpendicular magnetic
anisotropy (PMA) in an overlying laminated layer; and (b) a layer having
intrinsic PMA and comprising pure Co or a laminated stack represented by
(Fe/V)n where n is the number of laminates in the stack, and the Co
or (Fe/V)n laminated stack contacts a top surface of the seed layer.

26. The magnetic element of claim 25 wherein the seed layer and Co layer
or Fe/V)n laminated stack are formed in a magnetic tunnel junction
(MTJ) having a seed layer/reference layer/tunnel barrier/free layer
configuration, the Co layer or (Fe/V)n laminated stack is part of
the reference layer.

27. The magnetic element of claim 26 wherein the reference layer further
comprises a magnetic layer formed between the Co layer or (Fe/V)n
laminated stack and the tunnel barrier to increase a magnetoresistive
(MR) ratio in the MTJ, the magnetic layer interfaces with the tunnel
barrier and has PMA with a magnetic moment in a same direction as the PMA
in the Co layer or (Fe/V)n laminated stack.

28. The magnetic element of claim 25 wherein the seed layer and Co layer
or (Fe/V)n laminated stack are formed in a MTJ having a reference
layer/tunnel barrier/free layer/spacer/seed layer/dipole layer
configuration and the Co layer or (Fe/V)n laminated stack is the
dipole layer.

29. The magnetic element of claim 25 wherein the seed layer and the Co
layer or (Fe/V)n laminated stack are formed in a MTJ having a seed
layer/composite free layer/tunnel barrier/reference layer configuration
and the Co layer or (Fe/V)n laminated stack is part of the composite
free layer.

30. The magnetic element of claim 29 wherein the composite free layer
further comprises a magnetic layer formed between the Co layer or
(Fe/V)n laminated stack and the tunnel barrier to increase a
magnetoresistive (MR) ratio in the MTJ, the magnetic layer interfaces
with the tunnel barrier and has PMA with a magnetic moment in a same
direction as the PMA in the Co layer or (Fe/V)n laminated stack.

31. A method of forming a magnetic tunnel junction (MTJ); comprising: (a)
forming a seed layer on a substrate, the seed layer consists of one or
more of Hf, NiCr, and NiFeCr, and enhances perpendicular magnetic
anisotropy (PMA) in an overlying laminated layer; and (b) forming the
laminated layer having instrinsic PMA that contacts a top surface of the
seed layer, the laminated layer has a (Co/X)n or (CoX)n
structure wherein X is one of V, Rh, Ir, Os, Ru, Au, Cr, Mo, Cu, Ti, Re,
Mg, and Si, CoX is a disordered alloy, and n is the number of laminates
in the stack.

32. The method of claim 31 wherein the MTJ has a bottom spin valve
configuration in which the seed layer, a composite reference layer, a
tunnel barrier layer, and a free layer are sequentially formed on the
substrate, and the laminated layer is part of the composite reference
layer, the composite reference layer is further comprised of a magnetic
layer formed between the laminated layer and the tunnel barrier layer,
the magnetic layer has PMA aligned in the same direction as the PMA in
the laminated layer.

33. The method of claim 31 wherein the MTJ has a top spin valve
configuration in which the seed layer, a composite free layer, a tunnel
barrier layer, and a reference layer are sequentially formed on the
substrate, and the laminated layer is part of the composite free layer,
the composite free layer is further comprised of a magnetic layer formed
between the laminated layer and the tunnel barrier layer, the magnetic
layer has PMA aligned in the same direction as the PMA in the laminated
layer.

34. The method of claim 31 wherein the MTJ has a bottom spin valve
configuration in which a reference layer, a tunnel barrier layer, a free
layer, a non-magnetic spacer, and a dipole layer are sequentially formed
on a substrate, the seed layer and laminated layer are part of the dipole
layer.

Description:

[0001] This is a continuation in part of U.S. patent application Ser. No.
13/068,398, filed on May 10, 2011, which is herein incorporated by
reference in its entirety, and assigned to a common assignee.

FIELD OF THE INVENTION

[0002] The invention relates to a magnetic device that employs a thin film
made of a Ni/Co laminate or the like with a magnetization direction which
is perpendicular to the plane of the film (perpendicular magnetic
anisotropy or PMA) wherein PMA is enhanced by an improved seed layer that
induces a strong (111) crystal structure in the Ni/Co multilayer stack.

BACKGROUND OF THE INVENTION

[0003] Magnetoresistive Random Access Memory (MRAM), based on the
integration of silicon CMOS with magnetic tunnel junction (MTJ)
technology, is a major emerging technology that is highly competitive
with existing semiconductor memories such as SRAM, DRAM, and Flash.
Similarly, spin-transfer (spin torque or STT) magnetization switching
described by C. Slonczewski in "Current driven excitation of magnetic
multilayers", J. Magn. Magn. Mater. V 159, L1-L7 (1996), has stimulated
considerable interest due to its potential application for spintronic
devices such as spin-torque MRAM on a gigabit scale.

[0004] Both MRAM and STT-MRAM may have a MTJ element based on a tunneling
magneto-resistance (TMR) effect wherein a stack of layers has a
configuration in which two ferromagnetic layers are separated by a thin
non-magnetic dielectric layer. The MTJ element is typically formed
between a bottom electrode such as a first conductive line and a top
electrode which is a second conductive line at locations where the top
electrode crosses over the bottom electrode. A MTJ stack of layers may
have a bottom spin valve configuration in which a seed layer, an
anti-ferromagnetic (AFM) pinning layer, a ferromagnetic "pinned" layer, a
thin tunnel barrier layer, a ferromagnetic "free" layer, and a capping
layer are sequentially formed on a bottom electrode. The pinned or
reference layer has a magnetic moment that is fixed in the "y" direction,
for example, by exchange coupling with the adjacent anti-ferromagnetic
(AFM) layer that is also magnetized in the "y" direction. The free layer
has a magnetic moment that is either parallel or anti-parallel to the
magnetic moment in the pinned layer. The tunnel barrier layer is thin
enough that a current through it can be established by quantum mechanical
tunneling of conduction electrons. The magnetic moment of the free layer
may change in response to external magnetic fields and it is the relative
orientation of the magnetic moments between the free and pinned layers
that determines the tunneling current and therefore the resistance of the
tunneling junction. When a sense current is passed from the top electrode
to the bottom electrode in a direction perpendicular to the MTJ layers, a
lower resistance is detected when the magnetization directions of the
free and pinned layers are in a parallel state ("0" memory state) and a
higher resistance is noted when they are in an anti-parallel state or "1"
memory state.

[0005] As the size of MRAM cells decreases, the use of external magnetic
fields generated by current carrying lines to switch the magnetic moment
direction becomes problematic. One of the keys to manufacturability of
ultra-high density MRAMs is to provide a robust magnetic switching margin
by eliminating the half-select disturb issue. For this reason, the spin
torque MRAM was developed. Compared with conventional MRAM, spin- torque
MRAM has an advantage in avoiding the half select problem and writing
disturbance between adjacent cells. The spin-transfer effect arises from
the spin dependent electron transport properties of
ferromagnetic-spacer-ferromagnetic multilayers. When a spin-polarized
current transverses a magnetic multilayer in a CPP configuration, the
spin angular moment of electrons incident on a ferromagnetic layer
interacts with magnetic moments of the ferromagnetic layer near the
interface between the ferromagnetic and non-magnetic spacer. Through this
interaction, the electrons transfer a portion of their angular momentum
to the ferromagnetic layer. As a result, spin-polarized current can
switch the magnetization direction of the ferromagnetic layer if the
current density is sufficiently high, and if the dimensions of the
multilayer are small. The difference between a spin-torque MRAM and a
conventional MRAM is only in the write operation mechanism. The read
mechanism is the same.

[0006] For MRAM and spin-torque MRAM applications, it is often important
to take advantage of PMA films with a large and tunable coercivity field
(Hc) and anisotropy field (Hk). For example, PMA films may be used as a
pinned layer, free layer, or dipole (offset compensation) layer in a MTJ
element or in PMA media used in magnetic sensors, magnetic data storage,
and in other spintronic devices. Furthermore, a critical requirement is
that Hc, Hk, and other properties such as the magnetoresistive (MR) ratio
do not deteriorate during processing at elevated temperatures up to the
300° C. to 400° C. range. In some applications, it is also
necessary to limit the overall thickness of the PMA layer and
underlayers, and use only materials that are compatible with device
design and processing requirements.

[0007] Materials with PMA are of particular importance for magnetic and
magnetic-optic recording applications. Spintronic devices with
perpendicular magnetic anisotropy have an advantage over MRAM devices
based on in-plane anisotropy in that they can satisfy the thermal
stability requirement and have a low switching current density but also
have no limit of cell aspect ratio. As a result, spin valve structures
based on PMA are capable of scaling for higher packing density which is
one of the key challenges for future MRAM applications and other
spintronic devices.

[0008] When the size of a memory cell is reduced, much larger magnetic
anisotropy is required because the thermal stability factor is
proportional to the volume of the memory cell. Generally, PMA materials
have magnetic anisotropy larger than that of conventional in-plane soft
magnetic materials such as NiFe or CoFeB. Thus, magnetic devices with PMA
are advantageous for achieving a low switching current and high thermal
stability.

[0009] Several PMA material systems have been proposed and utilized in the
prior art such as multilayers of Pt/Fe, Pd/Co, and Ni/Co, and ordered
(e.g., L10 structures) as well as unordered alloys but there is still a
need for improvement in Hc, Hk, temperature stability, and material
compatibility. There is a report by M. Nakayama et al. in "Spin transfer
switching in TbCoFe/CoFeB/MgO/CoFeB/TbCoFe magnetic tunnel junctions with
perpendicular magnetic anisotropy", J. Appl. Phys. 103, 07A710 (2008)
related to spin transfer switching in a MTJ employing a TbCoFe PMA
structure. However, in a MTJ with a TbCoFe or FePt PMA layer, strenuous
annealing conditions are usually required to achieve an acceptably high
PMA value. Unfortunately, high temperatures are not so practical for
device integration.

[0010] Although (Co/Pt)x laminates are capable of generating high
PMA, Co/Pt and similar multilayers including Co/Pd and Co/Ir and alloys
thereof such as CoCrPt are not always desirable as a PMA layer in a MTJ
element because Pt, Pd, Ir, and Cr are severe spin depolarizing materials
and will seriously quench the amplitude of spintronic devices.

[0011] Among the PMA material systems studied, a Ni/Co multilayer is one
of the more promising candidates because of its large potential Hc and
Hk, good stability at high anneal temperatures, and potential
compatibility with other materials used in magnetic devices. However,
Ni/Co multilayers typically require a thick seed layer to induce high
PMA. A 500 Angstrom Ti or 500 Angstrom Cu seed layer with heating to
150° C. is used by P. Bloemen et al. in "Magnetic anisotropies in
Co/Ni (111) multilayers", J. Appl. Phys. 72, 4840 (1992). W. Chen et al.
in "Spin-torque driven ferromagnetic resonance of Co/Ni synthetic layers
in spin valves", Appl. Phys. Lett. 92, 012507 (2008) describe a 1000
Angstrom Cu/200 Angstrom Pt/100 Angstrom Cu composite seed layer. The
aforementioned seed layers are not practical with Ni/Co multilayer PMA
configurations in spintronic devices. Typically, there is a space
restriction in a direction perpendicular to the planes of the spin valve
layers in advanced devices in order to optimize performance. Seed layers
thicker than about 100 Angstroms will require thinning a different layer
in the spin valve structure to maintain a certain minimum thickness for
the MTJ element which can easily lead to performance degradation.

[0012] A [(Co/Ni)20] laminated structure with a thin Ta/Ru/Cu seed
layer is disclosed as a hard bias layer in U.S. Patent Application
Publication 2010/0330395 and as a reference layer in U.S. Patent
Application Publication 2009/0257151. However, even higher Hc is
desirable to be competitive with Hc values obtained with Pt/Co and Pd/Co
laminates.

[0013] In U.S. Pat. No 7,843,669, a fcc (111) crystal orientation is
described as desirable for a pinned layer or free layer but a Ni/Co
laminate with (111) orientation is not disclosed.

[0014] U.S. Pat. No. 7,190,613 describes a fixed layer having a high
coercive force and made of ordered alloys such as FePt, CoPt, and FePd,
or disordered alloys including Co/Cr, Co/Pt, Co/Cr/Pt and the like. For
ordered alloys with a fct (001) orientation, an underlayer such as MgO,
Pt, Pd, Au, Ag, Al or Cr with a similar crystal structure is preferred.

[0015] An improved seed layer is still needed that is thin enough to be
compatible with spintronic devices, can induce high PMA in overlying
Co/Ni multilayer structures, and is compatible with the design and
processing requirements of magnetic devices.

SUMMARY OF THE INVENTION

[0016] One objective of the present invention is to provide a seed
layer/PMA layer configuration for a magnetic device that has higher Hk
and Hc than previously realized and with thermal stability up to
400° C. process temperatures.

[0017] A second objective of the present invention is to provide a
material set for a high PMA structure according to the first objective
that is compatible with other layers in the magnetic device and has a
seed layer thickness of about 100 Angstroms or less.

[0018] According to one embodiment, these objectives are achieved with a
magnetic element that is a MTJ having a bottom spin valve configuration
in which a seed layer, PMA reference layer, tunnel barrier, free layer,
and capping layer are sequentially formed on a substrate. The seed layer
(underlayer) is preferably NiCr, NiFeCr, Hf, or a composite with a
Hf/NiCr, Hf/NiFeCr, NiFeCr/Hf, or NiCr/Hf configuration that induces a
strong (111) texture in the overlying Co/Ni multilayer stack within the
reference layer. In an alternative embodiment, Ni in the Co/Ni multilayer
is replaced by X to give a Co/X multilayer stack within the reference
layer where X is one of V, Rh, Ir, Os, Ru, Au, Cr, Mo, Cu, Ti, Re, Si,
and Mg. Preferably, the reference layer has a [(Co/Ni)n/CoFeB] or
[Co/X)n/CoFeB] configuration where n is from 2 to 30 and the CoFeB
(or CoFe) layer serves as a transitional layer between the (111)
crystalline structure of the Co/Ni or Co/X multilayer stack and the (100)
texture of a MgO tunnel barrier. The transitional layer preferably has
PMA and a magnetization aligned in the same direction as the PMA layer.
The free layer may be comprised of CoFeB, CoFe, or a combination thereof.
Thus, a high MR ratio is achieved together with high PMA in the reference
layer to enable greater thermal stability in the magnetic element.

[0019] In a second embodiment, the MTJ has a top spin valve structure
wherein a seed layer, PMA free layer, tunnel barrier, reference layer,
and capping layer are sequentially formed on a substrate. The PMA free
layer may be a composite with a Co/Ni or Co/X laminate formed on the seed
layer and an upper magnetic layer contacting the tunnel barrier. Again,
the magnetic layer may be CoFeB and serve as a transitional layer between
a (100) MgO tunnel barrier and a Co/Ni multilayer with a (111) texture.
Both of the first and second embodiments may further comprise a Ta
insertion layer between the Co/Ni or Co/X multilayer and the transitional
layer to prevent premature crystallization of the transitional layer
before the tunnel barrier is fabricated.

[0020] In a third embodiment, a dipole layer with an underlayer/PMA layer
configuration as defined in the first embodiment is used to provide an
offset field to an adjacent free layer. The MTJ has a stack represented
by seed layer/reference layer/tunnel barrier/free layer/spacer/dipole
layer/capping layer. The spacer may be a non-magnetic Ta layer to getter
oxygen from the free layer.

[0021] Once all the layers in the MTJ stack are laid down, a high
temperature annealing of about 350° C. may be employed to increase
the PMA within the Co/Ni laminated portion of the reference layer, free
layer, or dipole layer.

[0022] Alternatively, the (Co/Ni)n multilayer in the previous
embodiments may be replaced by a CoFe/Ni, Co/NiFe, Co/NiCo, CoFe/NiFe, or
a CoFe/NiCo laminated structure, by pure Co, or may be one of Co/Pt,
Co/Pd, Fe/Pt, Fe/Pd, or Fe/V.

[0023] In yet another embodiment, the (Co/X) stack described previously
may be replaced by a disordered alloy that is one of CoV, CoRh, CoIr,
CoOs, CoRu, CoAu, CoCr, CoMo, CoCu, CoTi, CoRe, CoMg, or CoSi to give a
(CoX)n laminated PMA layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a cross-sectional view showing a magnetic element
including a composite reference layer with a laminated PMA layer/magnetic
transitional layer formed according to a first embodiment of the present
invention.

[0025] FIG. 2 is the magnetic element in FIG. 1 further comprising a Ta
insertion layer between the laminated PMA layer and the magnetic
transitional layer according to another embodiment of the present
invention.

[0026]FIG. 3 is cross-sectional view of a magnetic element including a
composite free layer formed according to a second embodiment of the
present invention.

[0027] FIG. 4 is a cross-sectional view of a magnetic element including a
composite dipole layer formed according to a third embodiment of the
present invention.

[0028] FIG. 5 is a cross-sectional view depicting an embodiment wherein a
reference layer according to the first embodiment is formed in a domain
wall motion device.

[0029] FIG. 6a is a plot illustrating magnetic properties measured
perpendicular to the film plane for Co/Ni multilayers grown on less than
optimal seed layers.

[0030] FIG. 6b is a plot illustrating magnetic properties measured
perpendicular to the film plane for Co/Ni multilayers grown on Hf or NiCr
seed layers according to an embodiment of the present invention.

[0031] FIGS. 7a, 7b are plots showing magnetic properties measured
perpendicular to the film plane and in-plane, respectively, for Co/Ni
multilayers grown on Hf/NiCr or NiCr/Hf seed layers according to an
embodiment of the present invention.

[0032]FIG. 8 shows MH curves measured in a direction perpendicular to the
film plane for a MTJ element comprised of a (Co/Ni)10 laminated
reference layer formed on a Hf/NiCr seed layer.

[0033] FIG. 9 shows MH curves measured in a direction perpendicular to the
film plane for a MTJ element comprised of a (Co/Ni)10 laminated
dipole layer formed on a NiCr seed layer according to an embodiment of
the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0034] The present invention is a composite structure with a seed
layer/PMA layer configuration in a magnetic element wherein the seed
layer induces a strong (111) crystalline structure in an overlying
(Ni/Co)n, (Co/X)n or (CoX)n multilayer where X is one of
V, Rh, Ir, Os, Ru, Au, Cr, Mo, Cu, Ti, Re, Mg, or Si thereby generating
high PMA in the laminated stack. Note that "seed layer" may be used
interchangeably with the term "underlayer" in the exemplary embodiments,
and (Ni/Co)n and (Co/Ni)n, for example, are used
interchangeably when referring to a laminated stack. Although only bottom
and top spin valve structures are depicted in the drawings, the present
invention also encompasses dual spin valves having an enhanced PMA layer
that is incorporated in one or more of a reference layer, free layer,
dipole layer, or pinned layer in MRAM, spin-torque-MRAM, domain wall
motion devices, and in other spintronic devices. Furthermore, the PMA
structure may be used as a medium in magnetic sensors or in magnetic data
storage applications.

[0035] A key feature of the present invention is a (Co/Ni)n or
(Co/X)n multilayer structure having PMA where the perpendicular
magnetic anisotropy of the aforementioned laminate arises from spin-orbit
interactions of the 3d and 4s electrons of Co and Ni (or Co and X) atoms.
Such interaction causes the existence of an orbital moment which is
anisotropic with respect to the crystal axes which are in (111)
alignment, and also leads to an alignment of the spin moment with the
orbital moment. PMA character is enhanced by the presence of an
appropriate seed layer (underlayer) also having a (111) texture. Ideally,
the seed layer has a composition which is compatible with other materials
in a magnetic element that contains the (Co/Ni)n or (Co/X)n
laminate, is compatible with processing temperatures up to about
400° C., and is thin enough so as not to adversely affect the
performance of the magnetic element.

[0036] Referring to FIG. 1, a seed layer/PMA multilayer configuration is
shown as part of a MTJ 20 with a bottom spin valve structure according to
a first embodiment of the present invention. Each of the layers in the
MTJ are formed in an (x, y) plane and each have a thickness in a z-axis
direction. A substrate 21 is provided that may be a bottom electrode
layer, for example, made of Ta or other conductive layers. Substrate 21
may be formed on a substructure (not shown) that includes dielectric and
conductive layers as well as transistors and other devices. A key feature
is the seed layer 22 formed on substrate 21. The seed layer 22 has a
thickness from 10 to 300 Angstroms, and preferably 10 to 100 Angstroms.
According to one embodiment, the seed layer is NiCr, NiFeCr, or Hf. For a
NiCr or NiFeCr seed layer, Cr content is between 35 and 45 atomic % Cr,
and preferably is 40 atomic % Cr. Alternatively, seed layer 22 may be a
composite with a Hf/NiCr, Hf/NiFeCr, NiFeCr/Hf, or NiCr/Hf composition
wherein the NiCr or NiFeCr layer has a greater thickness than the Hf
layer. As a result of the preferred seed layer composition, a (111)
texture is induced and fully established in a (Co/Ni)n multilayer or
the like grown on a top surface of the seed layer. Top surface in this
context is a surface facing away from substrate 21.

[0037] Above the seed layer 22 is a composite reference layer 30 that
according to one embodiment has a (Co/Ni)n/CoFeB, a
(Co/X)n/CoFeB, or (CoX)n/CoFeB configuration wherein a lower
PMA layer 23 has a (Co/Ni)n, (Co/X)n, or (CoX)n
composition in which n is from 2 to 30, and preferably between 4 and 10,
and CoX is a disordered alloy. PMA layer 23 contacts the seed layer while
a magnetic layer 24 made of CoFeB, CoFe, or a combination thereof is
formed as an interface between the PMA layer and the tunnel barrier 25.
Each of the plurality of Co layers in the PMA layer has a thickness from
1.5 to 4 Angstroms, and each of the plurality of Ni, X, or CoX layers has
a thickness from 4 to 10 Angstroms. In yet another embodiment, PMA layer
23 may be pure Co.

[0038] In another embodiment, the lower PMA layer 23 may be comprised of
two metals, a metal and an alloy, or two alloys having an (A1/A2)n
configuration where A1 is a first metal or alloy selected from one or
more of Co, Ni, and Fe that may be doped with B up to 50 atomic %, A2 is
a second metal or alloy selected from one or more of Co, Fe, Ni, Pt, and
Pd, n is the number of laminates in the (A1/A2)n stack, and A1 is
unequal to A2. It should be understood that the laminated (A1/A2)n
stack has intrinsic PMA and the seed layer 22 is employed to enhance the
PMA property. Thus, the PMA layer 23 may be comprised of (CoFe/Ni)n,
Co/NiFe)n, (Co/NiCo)n, (CoFe/NiFe)n, or (CoFe/NiCo)n
laminates, for example. Alternatively, the PMA layer may have a
(Co/Pt)n, Co/Pd)n, (Fe/Pt)n, (Fe/Pd)n, or
(Fe/V)n composition, or a combination of the aforementioned
laminates. In another embodiment, A2 is a second metal or alloy selected
from one or more of X, FeX, and CoX. In this case, the PMA layer 23 may
be comprised of (CoFe/X)n, Co/FeX)n, (Co/CoX)n,
(CoFe/FeX)n, or (CoFe/CoX)n laminates.

[0039] In yet another embodiment, the lower PMA layer 23 may be comprised
of two individual magnetic layers separated by a non-magnetic spacer (C)
providing anti-ferromagnetic (RKKY) coupling between the two magnetic
layers as in an A1/C/A2 configuration. In this embodiment, the
non-magnetic spacer is preferably Ru with a thickness of 3 to 20
angstroms.

[0040] The present invention also encompasses an embodiment wherein the
lower PMA layer 23 may be an alloy with a L10 structure of the form MT
wherein M is Rh, Pd, Pt, Ir, or an alloy thereof, and T is Fe, Co, Ni or
alloy thereof. Furthermore, the MT alloy may be doped with B up to 40
atomic %.

[0041] Preferably, the magnetic layer 24 has a thickness from about 6 to
14 Angstroms which is sufficiently thin to enable interfacial
perpendicular anisotropy to dominate the in-plane shape anisotropy field
and thereby generate PMA character therein. According to one embodiment,
PMA within magnetic layer 24 is achieved as a result of the interface
between a thin CoFeB (or CoFe) layer and a metal oxide layer in tunnel
barrier 25 that leads to a significant amount of interfacial
perpendicular anisotropy, and the magnetic moments of layers 23, 24 are
aligned in the same direction along the z-axis. The magnetic layer serves
as a transitional layer between the (111) texture in PMA layer 23 and a
(100) texture in tunnel barrier 25 and may also enhance the
magnetoresistive (MR) ratio. As the magnetic layer thickness becomes
closer to 6 Angstroms, PMA character is maximized, and as layer 24
thickness approaches 14 Angstroms, MR ratio is increased. Therefore, the
thickness of the magnetic layer may be adjusted between 6 and 14
Angstroms to tune both PMA magnitude and MR ratio.

[0042] There is a tunnel barrier layer 25 preferably made of MgO formed on
the composite reference layer 30. However, other tunnel barrier materials
such as AlOx, TiOx, and ZnOx may be employed. A MgO tunnel barrier layer
may be fabricated by depositing a first Mg layer on the magnetic layer
24, then performing a natural oxidation (NOX) process, and finally
depositing a second Mg layer on the oxidized first Mg layer. During a
subsequent annealing process, the second Mg layer is oxidized to afford a
substantially uniform MgO layer. If a low RA (resistance x area) value is
desired, the thickness and/or oxidation state of the tunnel barrier 25
may be reduced as appreciated by those skilled in the art.

[0043] A free layer 26 is formed on the tunnel barrier layer 25 and may be
a composite and comprised of the same composition as in magnetic layer
24. Preferably, CoFeB, CoFe, or other materials which produce a
combination of high MR ratio, good switching property, and low
magnetostriction are selected as the free layer.

[0044] The uppermost layer in the spin valve stack is a capping layer 27.
In one aspect, the capping layer is a composite with a lower layer 27a
made of Ta and an upper layer 27b that is Ru which is used to provide
oxidation resistance and excellent electrical contact to an overlying top
electrode (not shown). A substantial reduction in critical current
density (Jc) occurs in STT-MRAM applications when a thin Ru layer is
employed as a capping layer due to the strong spin scattering effect of
Ru. The Ta layer may be included to offer etch resistance in subsequent
processing steps. Optionally, other capping layer configurations may be
employed. For example, the capping layer 27 may be a single layer of Ta
or Ru, a composite with a Ru/Ta/Ru configuration, or a composite with a
lower oxide or nitride layer and an upper Ru layer such as MgO/Ru or
AlOx/Ru. An oxide as the lower layer in the capping layer may be
advantageously used to promote PMA in a thin free layer 26.

[0045] Referring to FIG. 2, the composite reference layer 30 of the first
embodiment is modified to include a Ta insertion layer 35 about 0.5 to 3
Angstroms thick, and preferably 1.5 Angstroms thick, sandwiched between
the PMA layer and an amorphous CoFeB magnetic layer 24 to prevent
crystallization of the CoFeB layer before a MgO tunnel barrier 25 is
formed thereon. As a result, crystallization of an amorphous CoFeB layer
during a subsequent annealing step is driven by the (100) MgO tunnel
barrier and a major portion (not shown) of the magnetic layer
crystallizes in a (100) state to maximize the MR ratio in the MTJ. It
should be understood that a thin region (not shown) of a CoFeB magnetic
layer 24 which adjoins the PMA layer 23 will have a (111) crystal
structure or remains amorphous but the thickness of this thin region may
be minimized by including the Ta insertion layer 35.

[0046] Referring to FIG. 3, a second embodiment is depicted that shows a
MTJ 40 wherein the seed layer/PMA layer configuration of the present
invention is incorporated in a composite free layer. In other words, the
seed layer/PMA layer configuration as defined herein is not limited to a
reference (pinned) layer but may be used as any functional or passive
layer in a magnetic stack. Preferably, the MTJ in this embodiment has a
top spin valve structure in which a seed layer, composite free layer,
tunnel barrier, reference layer, and a capping layer are sequentially
formed on substrate 21. The seed layer 22 and PMA layer 23 are retained
from the first embodiment but in this case the laminated PMA layer is
included in a composite free layer. Reference layer 29 formed on the
tunnel barrier is comprised of CoFeB, CoFe, or combinations thereof and
may have a well known synthetic anti-parallel (SyAP) configuration (not
shown) wherein two ferromagnetic layers such as CoFeB are separated by a
coupling layer which is Ru, for example. Tunnel barrier 25 and capping
layer 27 are retained from the first embodiment.

[0047] Preferably, the tunnel barrier is MgO to maximize the MR ratio in
the MTJ 40. Furthermore, the composite free layer 31 may have a seed
layer 22/PMA layer 23/magnetic layer 24 configuration where the magnetic
layer serves as a transitional layer between the (100) texture in the
tunnel barrier layer and the (111) crystal structure in PMA layer 23 to
promote a high MR ratio. Preferably, the magnetic layer 24 is made of
CoFeB with a thickness of about 6 to 14 Angstroms so that interfacial
perpendicular anisotropy dominates shape anisotropy within the layer to
result in PMA with a magnetization direction that is aligned in the same
z-axis direction as in PMA layer 23. The composite free layer 31 may
further include a Ta insertion layer (not shown) formed between PMA layer
23 and magnetic layer 24. Seed layer (underlayer) 22 is preferably one of
NiCr, NiFeCr, Hf, Hf/NiCr, Hf/NiFeCr, NiFeCr/Hf, or NiCr/Hf with a 5 to
200 Angstrom thickness to enhance the PMA character in PMA layer 23
thereby increasing thermal stability without compromising other free
layer properties.

[0048] Referring to FIG. 4, a third embodiment of the present invention is
illustrated and depicts a MTJ 50 having a bottom spin valve structure
wherein the seed layer/PMA layer of the present invention is employed as
a dipole layer to reduce the offset of the minor switching loop of the
free layer caused by a dipole field from the reference layer. According
to one embodiment, a seed layer, reference layer, tunnel barrier, free
layer, spacer, dipole layer, and a capping layer are sequentially formed
on substrate 21. The seed layer 28 may be NiCr or NiFeCr. Reference layer
29 and tunnel barrier layer 25 are retained from the second embodiment.
Preferably, the reference layer is thin with a thickness from 5 to 15
Angstroms. Free layer 26 and capping layer 27 were previously described
with regard to the first embodiment. In one aspect, a CoFeB/MgO/CoFeB
reference layer/tunnel barrier/free layer may be employed to provide a
high MR ratio.

[0049] A key feature is the stack of layers formed between the free layer
and capping layer. In particular, a non-magnetic spacer 33 made of Ta,
for example, contacts a top surface of free layer 26 and getters oxygen
from the free layer. Above the spacer is a composite dipole layer 34
including an underlayer 22 that contacts a top surface of the spacer and
a PMA layer 23 that interfaces with capping layer 27. Layers 22, 23
retain the same features as described with respect to the first two
embodiments except the underlayer is preferably 5 to 100 Angstroms thick
in this embodiment. In one aspect, free layer 26 may be sufficiently thin
(6 to 15 Angstroms) to have significant interfacial perpendicular
anisotropy that dominates an in-plane shape anisotropy field such that a
magnetization perpendicular to the plane of the free layer is
established. Interfacial perpendicular anisotropy is a result of the
interface between a bottom surface of free layer 26 and a top surface of
tunnel barrier 25 which is preferably MgO. When the free layer has PMA,
the magnetization directions of the free layer and PMA layer 23 are
preferably aligned in the same direction.

[0050] All of the layers in the MTJ elements described herein may be
formed in a sputter deposition system such as an Anelva C-7100 thin film
sputtering system or the like which typically includes three physical
vapor deposition (PVD) chambers each having 5 targets, an oxidation
chamber, and a sputter etching chamber. At least one of the PVD chambers
is capable of co-sputtering. Typically, the sputter deposition process
involves an argon sputter gas with ultra-high vacuum and the targets are
made of metal or alloys to be deposited on a substrate. All of the MTJ
layers may be formed after a single pump down of the sputter system to
enhance throughput.

[0051] The present invention also encompasses an annealing step after all
layers in the spin valve structure have been deposited. The MTJ elements
20, 40, 50 may be annealed by applying a temperature between 300°
C. and 400° C. for a period of 30 minutes to 5 hours using a
conventional oven, or for only a few seconds when a rapid thermal anneal
oven is employed. No applied magnetic field is necessary during the
annealing step because PMA is established in layer 23 due to the seed
layer 22 and because of the Co--Ni (or A1-A2) spin orbital interactions
in the laminated PMA layer 23.

[0052] Once all the layers in MTJ elements 20, 40, or 50 are formed, the
spin valve stack is patterned into an oval, circular, or other shapes
from a top-down perspective along the z-axis by a well known photoresist
patterning and reactive ion etch transfer sequence. Thereafter, an
insulation layer (not shown) may be deposited on the substrate 21
followed by a planarization step to make the insulation layer coplanar
with the capping layer 27. Next, a top electrode (not shown) may be
formed on the capping layer.

[0053] Referring to FIG. 5, an embodiment is depicted wherein the MTJ of
the first embodiment is fabricated in a domain wall motion device. In one
aspect, composite reference layer 30 is formed in a MTJ stack having a
seed layer/reference layer/tunnel barrier/free layer/capping layer
configuration. A key feature is that seed layer 22 and reference layer 30
have a width along an in-plane x-axis direction that is substantially
less than the width of the overlying stack of layers. In fact, the stack
of layers including tunnel barrier 25, free layer 26, and capping layer
27 may be patterned to provide a wire which from a top-down view (not
shown) is part of an array of wires that are employed for storage of
digital information. Another important feature is that free layer 26 has
a plurality of domain walls (75a-75d) each extending vertically from a
top surface that interfaces with layer 27a to a bottom surface which
interfaces with tunnel barrier layer 25. There is a magnetic domain
bounded by each pair of domain walls within the composite free layer. The
number of domain walls is variable but is selected as four in the
exemplary embodiment for illustrative purposes. In particular, the
magnetic domain 92 aligned in a z-axis direction above reference layer 30
has a switchable magnetization that changes from a (+)z-direction to a
(-)z-direction or vice versa when a switching current is applied during a
write process. Note that free layer has two ends 37e, 37f connected to a
current/voltage source 81 in a first electrical loop including wiring 85a
to a junction 82 to wire 83 and to end 37e, and a wire 84 attached to end
37f to enable a write process. Furthermore, there is a second electrical
loop which allows a readout of digital information in the switchable
magnetic domain 92 during a read process. Thus, current can be sent from
source 81 through wires 85a, 85b and to readout 80 and then to wire 86
and through reference layer 30, tunnel barrier 25, and free layer 26
before exiting end 37f and returning to the source to complete a circuit.
In so doing, the readout device 80 is able to recognize whether the
switchable magnetic domain 92 has a magnetization in a (+)z-axis
direction 90b or in a (-)z-axis direction 90a.

EXAMPLE 1

[0054] An experiment was performed to demonstrate the advantage of a seed
layer in improving PMA in an overlying (Co/Ni)n multilayer stack
according to the present invention. A partial and unpatterned spin valve
stack comprised of a seed layer, a (Co/Ni)10 laminated layer where
each Co layer is 2.5 Angstroms thick and each Ni layer is 6 Angstroms
thick, and a Ru cap layer was fabricated in order to obtain PMA values
from M-H curves using a vibrating sample magnometer (VSM). All samples
were annealed at 350° C. for 1 hour. In FIG. 6a, less than ideal
seed layers such as TaN, PtMn, NiFe, and Ru were employed. In FIG. 6b,
100 Angstrom thick NiCr and Hf seed layers were formed according to the
present invention to provide improved performance including squarer M-H
loops and higher coercivity (higher PMA) as depicted in curves 41, 42,
respectively, compared with the films grown on the inadequate seed layers
in FIG. 6a. Note that a greater distance between the vertical sections of
each pair of curves in FIG. 6b means a higher PMA is achieved. Thus, a
NiCr seed layer leads to higher PMA than a Hf seed layer of similar
thickness.

[0055] Referring to FIGS. 7a-7b, additional partial spin valve stacks were
prepared by sequentially forming a composite seed layer, (Co/Ni)10
laminated layer, and Ru capping layer. Coercivity in an in-plane
direction with respect to the film plane is illustrated in FIG. 7b for a
Hf20/NiCr100 seed layer configuration (curve 53) and for a NiCr100/Hf20
seed layer configuration (curve 54). Magnetization or PMA in a
perpendicular-to-plane direction is represented for the Hf/NiCr and
NiCr/Hf configurations in curves 51, 52, respectively in FIG. 7a. The
resulting laminated films grown on composite seed layers of the present
invention show high coercivity and good squareness. Related measurements
on single layer seed films show a high saturation field (perpendicular
anisotropy) of about 4000 Oe for a Hf underlayer and around 10000 Oe for
a NiCr underlayer. Note that in a composite seed layer such as Hf/NiCr,
the upper layer which contacts the Co/Ni (or A1-A2) laminate has a larger
effect in enhancing PMA in the multilayer than the lower layer contacting
the substrate. Since a NiCr underlayer generates a higher PMA than a Hf
underlayer (10000 Oe vs. 4000 Oe) as indicated previously, a Hf/NiCr seed
layer (curve 51) leads to a higher PMA than a NiCr/Hf seed layer (curve
52)

EXAMPLE 2

[0056] To further demonstrate the benefits of the present invention
according to a first embodiment where the composite seed layer/laminated
PMA layer is formed as a reference layer in a MTJ suitable for
spin-torque MRAM devices, a MTJ stack was fabricated as represented in
the following stack of layers where the number following each layer is
the thickness in Angstroms:
Si/SiO2/Ta50/Hf20/NiCr100//(Co2.5/Ni6)10/Co20Fe60B.su-
b.206/MgO/Co20Fe60B2012/Ta20/Ru. In the aforementioned
structure, Si/SiO2 is the substrate, Ta is a bottom electrode,
Hf/NiCr is a composite seed layer, (Co/Ni)10 is the laminated PMA
portion of the reference layer, CoFeB is the transitional magnetic layer
adjoining a MgO tunnel barrier, CoFeB is the free layer, and Ta/Ru is the
capping layer. Thus, (Co/Ni)10/Co20Fe60B20 serves as
a composite reference layer to provide a high MR ratio wherein the CoFeB
portion is thin enough to preserve the PMA property and thick enough to
generate a high MR ratio because of cohesive tunneling in the
CoFeB/MgO/CoFeB stack of layers. In addition to promoting a high
magneto-resistive ratio, the CoFeB free layer is selected for switching
purposes. An anneal process comprising a 300° C. temperature
treatment for 1 hour was used for this experiment.

[0057] Referring to FIG. 8, a M-H loop measurement is illustrated for the
MTJ stack described above and shows significant PMA as a result of the
composite reference layer structure, and an additional PMA contribution
from the CoFeB free layer. The steps 61a, 61b in the M-H loop indicate
the reference layer and free layer switch independently with the
reference layer having a much greater coercivity compared with the free
layer as revealed by the greater height for step 61b related to the
reference layer. Therefore, the reference layer may serve as a "pinned
layer".

EXAMPLE 3

[0058] According to a third embodiment of the present invention, the
composite seed layer/laminated PMA layer as described previously may be
incorporated as a dipole layer in a MTJ represented in the following
stack of layers:
Si/SiO2/Ta50/NiCr100/Co20Fe60B203/MgO/Co20Fe.sub-
.60B2012/Ta10/NiCr 20/(Co2.5/Ni6)10/Ru. In the aforementioned
structure, Si/SiO2 is the substrate, Ta50 is a 50 Angstrom thick
bottom electrode, NiCr100 is a 100 Angstrom thick seed layer,
CoFeB/MgO/CoFeB is a reference layer/tunnel barrier/free layer
configuration, and Ta10 is non-magnetic spacer with a 10 Angstrom
thickness between the free layer and a 20 Angstrom thick NiCr underlayer
for the laminated PMA structure which is (Co/Ni)10. Ru is a capping
layer.

[0059] Referring to FIG. 9, a M-H loop measurement is depicted that shows
magnetization in a direction perpendicular to the film plane of the
layers in the aforementioned MTJ stack having a dipole layer. In this
case, the dipole layer and free layer both have PMA and switch at
different fields allowing the two layers to be set in the desired
configuration. Step 71b for the dipole layer is substantially greater
than the step 71a for the free layer. There is no PMA contribution
observed for the reference layer which is purposely kept very thin at 3
Angstroms for this experiment. In a functional MTJ, reference layer
thickness is generally maintained between 5 and 15 Angstroms and would
exhibit substantial PMA. By reducing the thickness of the Ta spacer
between the dipole layer and the free layer, the dipole layer is allowed
to provide more offset field to the free layer. Moreover, the thickness
of the NiCr underlayer adjoining the Co/Ni laminate may be adjusted to
amplify or reduce the offset field applied to the free layer.

[0060] The MTJ elements described herein feature a seed layer/PMA
laminated layer combination which offers enhanced PMA properties together
with improved compatibility with high temperature processing, and
improved compatibility with the design and fabrication of magnetic
devices. As a result, the embodiments of the present invention are
suitable for a variety of applications including advanced PMA spin-torque
MRAM devices, domain wall motion devices, and in-plane magnetic devices
wherein it is beneficial to introduce an out-of-plane magnetic anisotropy
component as in in-plane spin torque MRAM devices or in partial PMA spin
torque MRAM devices. Improved PMA properties include increased Hc and Hk
that enable higher thermal stability up to 400° C. processing.

[0061] While this invention has been particularly shown and described with
reference to, the preferred embodiment thereof, it will be understood by
those skilled in the art that various changes in form and details may be
made without departing from the spirit and scope of this invention.